Coatings for outdoor uses, for example, coatings for the exterior of automobiles, must be capable of providing a coated film whose surface appearance remains unchanged even when it is exposed to various weather conditions
As is well known, in the case where titanium oxides are contained in a coated film, polymers that constitute the flm tend to be oxidatively decomposed by ultraviolet rays and moisture, thereby causing whitening or so-called chalking of the coating. In order to suppress such an activity of titanium dioxide, the generally employed titanium dioxide pigments are stabilized by various techniques, for example, by covering titanium dioxide with chromium compounds, silicon compounds, aluminum compounds, zinc compounds, phosphorus compounds, zirconium compounds or a combination of these compounds, or by doping such compounds into titanium dioxide. Aluminum flake pigments have been used as a coating for the exterior of automobiles. However, as is well known, the appearance of coated films containing an aluminum flake pigment is liable to change when it is exposed to various weather conditions. As means for evaluating the applicability of this kind of pigment, there have been employed the outdoor exposure test, accelerated weathering test, or the like. In particular, in order to examine the weather resistance of coated films, coated plates are exposed to high temperature high humidity conditions (blister box test) or immersed in hot water (hot water immersion test), and the deterioration in gloss and the change in color hue of the coated films are examined.
The cause of the deterioration in gloss and the change in color hue is believed to be due to water or water vapor penetrated into coated films. When this happens, micro-blisters are formed. The subsequent evaporation of water therefrom leaves holes in the coated films, and the thus-formed holes scatter light.
Nacreous pigments composed of mica flakes covered with metal oxides, such as titanium oxides, iron oxides or a combination of titanium oxides and iron oxides, have been used as coloring agents for coatings, inks, plastics, etc., in various applications, such as daily necessaries, toys, wrapping materials and the like. Recently, their uses have extended to outdoor applications, including the exterior coatings of automobiles, building materials, and the like.
However, in water resistance tests, such as the aforementioned blister box test and hot water immersion test, of hitherto known nacreous pigments, there were observed substantial deteriorations in gloss and changes in color, similar to those observed in aluminum flake pigments. It is therefore pointed out that the known nacreous pigments are not suited for coatings of outdoor use, in particular, for exterior coatings of automobiles.
In Japanese Patent Publication No. 4,147/80 are described metal oxide-coated mica pigments which were stabilized against severe weather conditions by means of a surface treatment using chromium compounds.
It is however disadvantageous to use such pigments treated with chromium compounds. Upon coating operations, part of these coatings fails to attach on articles to be coated. Therefore, if the coatings contain chromium-treated pigments, waste water from painting plants must be treated before being discharged to rivers or the like since, e.g., in Japan, chromium is placed under strict control by water pollution prevention law as a poisonous substance.
In addition, chromium compounds have deep green to grayish green color inherent thereto and, therefore, they are not suited for the production of, e.g., white nacreous pigments.
Most ordinary pigments are produced in the form of an agglomerated mass, which is then ground into powders having a usable granularity. On the other hand, nacreous pigments must not be pulverized or allowed to agglomerate from the time of their production to their actual use since coated films having a good nacreous luster can be obtained only when nacreous pigments are dispersed in the form of primary particles and arranged parallel to the surface of the coated films while maintaining their flakelike shapes. In other words, ordinary pigments are capable of providing sufficient color effects even when they are not in the state of primary particles, whereas it is absolutely necessary for nacreous pigments to be in the state of primary particles. However, it has not been possible to obtain nacreous pigments stabilized in the state of primary particles since they form agglomerates when stabilization techniques for ordinary pigments are applied thereto.
In the field of automobiles, demand for nacreous finishing has been increasing in recent years. It has been strongly desired in this field to develop nacreous pigments for outdoor use having both excellent gloss and excellent water resistance, without the use of chromium compounds.
The present invention provides a novel water-resistant nacreous pigment that meets the above requirements and a process for producing the same.